Helper-dependent adenoviruses (HDAd) can transduce hepatocytes to direct high level, long-term transgene expression with no chronic toxicity. However, high doses are required for efficient transduction following systemic administration which results in an acute inflammatory response which can lead to severe and lethal acute toxicity. This is the most significant obstacle hindering clinical progress of this otherwise promising technology. The severity of this response appears to be 1) clearly dose-dependent and 2) related to systemic vector dissemination. Therefore, we hypothesize that this acute toxicity can be minimized/eliminated by delivering the vector exclusively/preferentially to the liver to permit high efficiency hepatic transduction with very low doses and with no/minimal systemic vector dissemination. Specific Aim 1 is to ascertain whether there is a direct correlation between hepatic transduction and the severity of acute toxicity in mice. This is important because it directly relates to the clinical reality of liver-directed gene therapy using HDAd. Our preliminary results suggest that systemic vector dissemination, rather than hepatic transduction plays a major role in acute inflammatory response activation. Specific Aim 2 is to investigate the safety, feasibility and efficacy of delivering HDAd exclusively into the liver by developing minimally invasive, clinically relevant, percutaneous approaches to achieve liver exclusive vector delivery in baboons using balloon occlusion catheters. Our preliminary results in mice and baboons suggest that liver exclusive vector delivery will permit efficient hepatic transduction with low doses thus minimizing systemic dissemination and acute toxicity. Specific Aim 3 is to investigate the long term effects of liver exclusive vector delivery in baboons, such as duration of transgene expression, chronic toxicity and readministration. Specific Aim 4 is to develop a method of mimicking hydrodynamic injection of HDAd in baboons that is noninvasive and does not require large injection volumes. Our preliminary results in a baboon suggest that this novel delivery method has the potential of dramatically increasing the vector's therapeutic index by permitting exceedingly high, unprecedented levels of hepatic transduction with low vector doses and with reduced systemic vector dissemination and proinflammatory cytokines. Successful completion of these studies may finally permit safe and efficacious HDAd-mediated, liver-directed gene therapy for a wide variety of genetic diseases.